The present invention satisfies the need of constructing catoptric optical systems from sheets of reflective materials having low tensility.
Information relevant to attempts to address these problems can be found in U.S. Pat. Nos. 3,511,559; 4,429,952; 5,729,387; and 6,667,831; which are not admitted to be prior art with respect to the present invention by their mention in this Background Section. It is desirable to have better apparatuses and/or methods than what is disclosed in the identified references.
There is a class of reflective sheet materials that, although otherwise suitable for optical reflection systems, cannot be used to construct standard parabolic dish reflectors because they have low tensility: they cannot be pulled or stretched without causing mechanical stress leading to fracture. Alanod® is one such material. A sheet of Alanod® cannot be formed into a parabolic dish without causing mechanical stress leading to fracture, unless the dish is very shallow, which limits its utility for solar applications. These materials however are capable of simple bends, such as bending a sheet in one direction into a trough. Such a constraint motivates the question whether already known or classic dish-based optical systems with parallel output beams can be constructed from troughs instead. The present invention answers this question affirmatively. For want of a better term we say it is a “Mersenne-like” reflector system, with confocal parabolic optical surfaces, that replaces the usual Mersenne parabolic dishes with parabolic troughs but otherwise retains the Cassegrainian or Gregorian configuration of those optical elements (dishes, troughs). It is “Mersenne-like” rather than “Mersenne” because it uses troughs rather than dishes. It is useful for at least daylighting systems. In replacing the Mersenne parabolic dish design with a parabolic trough design, in order to maintain the same flux collection area with a fixed diameter emerging collimated beam, the trough is elongated in the curving direction, producing an elongated mirror with a longitudinal axis connecting the ends of the long direction of the mirror, these end points herein called “tips.”
Reflector telescopes made from coaxial, confocal paraboloids were first described by Mersenne in the year 1636. Mersenne invented an afocal reflecting telescope that utilizes a larger perforated primary parabolic dish that reflects light onto a smaller secondary parabolic dish; the secondary then reflects light back through the perforation in the primary. In the Cassegrainian form of the Mersenne system the primary reflector is concave, the secondary reflector is convex, and the secondary is placed at a distance from the primary that is shorter than the focal length of the primary. In the Gregorian form of the Mersenne system the secondary reflector is also concave, but it is moved to a distance longer than the focal length of the primary. The confocal aspect of these systems makes them afocal: a parallel or collimated beam entering the system will be parallel or collimated leaving the system. As pointed out in U.S. Pat. No. 5,471,346, the configurations and relative positions of the primary and secondary reflectors or mirrors also determine obscuration ratios, f-numbers, and the overall focal length (or afocal magnification) of the system. These systems utilize concave primary and smaller convex secondary “paraboloids,” or parabolas of revolution about an axis (herein called “dishes”). In fact, Mersenne reflectors strictly utilize parabolic dishes. Other reflector telescopes utilize other surfaces of revolution based on conic sections such as hyperbolas and circles, yielding hyperboloids and spheres, but they all utilize dishes.
The present invention is designed for concentrating direct-beam sunlight for general illumination and/or energy concentration uses, not for imaging, so the imaging constraints of the traditional Mersenne system are not needed. In some embodiments, the present invention truncates the parabolic troughs by splitting them through their centers and perpendicular to their longitudinal axes. Other inventions also truncate paraboloids, but what distinguishes some of the embodiments of the present invention is that they rejoin the parabolic troughs following truncation, resulting in a geometry that is no longer strictly parabolic but truncated: converted into two sections of a parabolic sheet. In the Cartesian coordinate system used herein, these three axes are perpendicular to each other: longitudinal, transverse, and optical.